The walnut originated on the shores of the Caspian Sea and has been cultivated for thousands of years. The Romans considered the nut sacred because the tree it grows on lives so long, and some believe that walnuts cure headaches because they are shaped like a human brain. Not only is the walnut used for food, and to heal headaches, but also its shell is used to make a dye for wood furniture. Walnuts are a rich source of the minerals copper and magnesium. They are also good sources of potassium, vitamin B6, folic acid, thiamin, fiber, niacin, iron, riboflavin, and pantothenic acid. Keep shelled walnuts in the refrigerator to prevent them from going rancid.